The present invention relates to optoelectronic devices such as light-emitting diodes. In particular, the present invention relates to light-emitting diodes having an optimized contact geometry in a flip-chip configuration.
Light-emitting diodes (“LEDs”) may include thin layers of semiconductor material of two opposite conductivity types, referred to as p-type and n-type. The layers may be disposed in a stack on a substrate. The stack may include one or more layers of n-type material in one part of the stack and one or more layers of p-type material in another part of the stack. The stacked material, including the substrate, may form a wafer. The wafer may be cut apart to form individual dies constituting separate LEDs. The junction between the p-type and n-type material (“the p-n junction”) may include directly abutting p-type and n-type layers, or may include one or more intermediate layers that may be of any conductivity type.
In operation, electric current passing through an LED is carried principally by electrons in the n-type layers and by electron vacancies or “holes” in the p-type layers. The electrons and holes move in opposite directions toward the junction, and recombine with one another at the junction. Energy released by electron-hole recombination is emitted as light. As used in this disclosure, the term “light” radiation includes infrared and ultraviolet wavelength ranges, as well as the visible range. The wavelength of the light depends on factors including the composition of the semiconductor materials and the structure of the junction.
Electrodes may be connected to the n-type and p-type layers. The materials in the electrodes are selected to provide low-resistance interfaces with the semiconductor materials. The electrodes, in turn, are provided with pads suitable for connection to wires or other conductors that carry current from external sources. The pads may transfer heat away from the LED. The pad associated with each electrode may be a part of the electrode, having the same composition and thickness of the electrode, or may be a distinct structure that differs in thickness, composition, or both from the electrode itself. The term “electrode-pad unit” is used herein to refer to the electrode and pad, regardless of whether the pad is a separate structure or merely a region of the electrode.
LEDs formed from certain semiconductor materials normally use nonconductive substrates to promote proper formation of the semiconductor layers. The nonconductive substrate typically is left in place, so that an electrode cannot be provided on the bottom surface of the bottom layer. For example, gallium nitride-based materials such as GaN, AlGaN, InGaN and AlInGaN, are used, as well as aluminum nitride (AlN), alumina and zinc oxide (ZnO) to form LEDs emitting light in various wavelength ranges including blue and ultraviolet. These materials typically are grown on insulating substrates such as sapphire or alumina.
LEDs incorporating an insulating substrate must include a bottom electrode at a location on the stack above the substrate but below the junction. Typically, the upper layer or layers of the stack are removed in a region covering part of the area of each die after formation of the stack, so as to provide an upwardly-facing lower electrode surface on a layer at or near the middle of the stack in each die. This leaves a region referred to as a “mesa” projecting upwardly from the lower electrode surface and covering the remaining area of the die. The area of the die occupied by the lower electrode surface does not emit light. It is desirable to keep the horizontal extent of this inactive area as small as possible.
The top electrode typically is formed on the top surface of the stack, i.e., the top surface of the top semiconductor layer. Typically, the layers in the stack above the junction are transparent, so that light emitted at the junction can pass out of the stack through the top surface. The top electrode is arranged so that it does not block all of the emitted light. For example, an opaque top electrode may cover only a small portion of the top surface of each die. However, the current passing from such an electrode will tend to flow downwardly through the stack so that the current passes predominantly through the area of the junction disposed beneath the electrode. This phenomenon, referred to as “current crowding,” results in light emission concentrated in that area of the junction beneath the electrode, precisely where it will be most effectively blocked by the electrode. The amount of useful light reaching the outside of the die per unit of electrical current passing through the die, commonly stated as the external quantum efficiency of the die, is reduced by this phenomenon. Current crowding can also occur in the lower region, so that light emission is concentrated in the area of the junction near the lower electrode. Current crowding is a significant consideration with LEDs formed from materials having relatively high electrical resistivity, such as the gallium nitride-based materials. In addition to current crowding, heat dissipation is also a significant consideration for high powered LEDs.
To alleviate the current crowding problem, LEDs have been provided with electrodes that promote “current spreading” by dispersing current laterally over the p-type material and the n-type material. In a “top emitting” die, the top or p-electrode may be transparent and may extend over substantially the entire top surface of the die. The top electrode is provided with a relatively small, opaque pad for connection to external circuitry. However, even a nominally transparent electrode will absorb some of the light emitted in the die. A thicker top electrode, which provides more effective current spreading, aggravates this problem. Such a thick electrode promotes a low spreading resistance across the p-type material. A similar solution could be used for the n-type electrode. However, areas covered by the n-type electrode will not emit light, so such areas should be minimized.
Other LEDs are mounted in a “flip-chip” arrangement, with the top surface of the LED facing toward the mounting and with the substrate facing away from the mounting. The substrate is transparent to light at the emission wavelength of the LED, so that emitted light can pass out of the LED through the substrate. The light will pass through transparent stack layers and be emitted from a transparent substrate. The term “transparent substrate” is used herein to refer to a material that has an absorption coefficient on the order of 10 cm−1 or less at the emission wavelength of the LED.
Various proposals have been advanced for achieving a good balance between current spreading and light blockage by the electrodes in a top-emitting die. One design for promoting current spreading is shown in U.S. Pat. No. 6,307,218. That design includes one electrode partly or wholly surrounding the other electrode, when examined from a top plan view. Current spreading may also be performed by a design having an outer electrode substantially surrounding the edges of the top surface with an outer electrode. The outer electrode may have one or more arms disposed so that the arms surround the light-emitting region of the LED.
Although the U.S. Pat. No. 6,307,218 patent states that the same electrode designs used in a top-emitting die can be employed in a flip-chip die, this would not lead to an optimum solution. Considerations such as current spreading, loss of active die area to areas occupied by the n-electrode and blockage of light emission by electrodes and pads are different in a flip-chip die. Moreover, considerations relating to heat extraction from the die in flip-chip designs are different from those encountered in top-emitting designs. Therefore, a need exists for improved LED designs.